This invention relates to a fluidized bed reactor and method of operating same and, more particularly, to such a reactor and method in which heat is generated by the combustion of particulate fuel in a fluidized bed.
Fluidized bed reactors, usually in the form of combustors, boilers, gasifiers, or steam generators, are well known. In a normal fluidized bed arrangement, air is passed through an air distribution grid, which supports a bed of particulate material, usually including a mixture of fuel material, such as high sulfur bituminous coal, fuel ash, and an adsorbent material for the sulfur released as a result of the combustion of the coal. As a result of the air passing through the bed, the bed behaves like a boiling liquid which promotes the combustion of the fuel. In addition to enjoying a capability for considerably reducing the amount of sulfur-containing gases introduced to the atmosphere, such an arrangement permits relatively high heat transfer rates per unit size, substantially uniform bed temperatures, relatively low combustion temperatures, ease of fuel handling, reduction in corrosion and boiler fouling, and the generation of a relatively low volume of nitrous oxides. To maintain the bed temperature at the desired level, usually between 1400-1800.degree. F., heat has to be removed from the bed. This is usually accomplished by inserting tubes into the bed through which water, steam or air is circulated and heated. For good combustion, the amount of air introduced through the air distribution grid is on the order of 20 percent higher than theoretically necessary. Furthermore, the fuel introduced into the bed has to be uniformly distributed over the bed plan area to prevent formation of pockets of reducing atmosphere. However, these units, which normally operate with fluidizing velocities of 5-10 ft./sec., have a number of drawbacks.
For example, combustion efficiency decreases as the fluidizing velocity increases. To improve this, fines recirculation is required which is costly and difficult to operate. Also, in-bed heat transfer tubes are subject to erosion which increases as the fluidizing velocity increases and a large quantity of limestone is required to capture the sulfur dioxides generated from the sulfur in the fuel. Further, it is difficult to obtain uniform fuel distribution over the cross-section area of the bed and previous attempts to do this has, in many instances, proven to . be unreliable.